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W. A. Harvey and W. W. Robblns 



Arrowhead lily 
Austrian field cress* 
Black medic 
Blue lettuce* 
Bull thistle 

California mugwort 
Canada jdeabane 
Cattail (young)* 
Creek nettle 
Curly dock 
Hoary cress 


Indian strawberry 


Klamath weed (St. John's 

Leafy spurge* 
Milk thistle 
Mouse-ear chickweed 

Oxalis (green)* 
Perennial dogbane* 
Perennial ragweed* 
Poison hemlock 
Prostrate pigweed 
Puncture vine 
Red clover 
Rough pigweed 
Russian thistle (young) 


Sheep sorrel* 

Shepherd's purse 

Sow thistle (annual) 

Spiny clotbur 

Spotted spurge 

Star thistles (rosette) 




Tumbling pigweed 

Water hemlock* 

Water hyacinth 

Water plantain 

Water primrose 

Western ragweed 

White horse nettle* 

Wild carrot 

Wild lettuce 

Wild morning-glory^^ 

Wild radish 

Wild sunflower 


* Those marked by an asterisk (*) may require two or more treatments. 

Species Resistant to 2,4-D 

Alkali mallow 
Annual bluegrass 
Baby tears 
Bermuda grass 
Bracken fern 
Button willow 
Canada thistle 
Crab grass 
Dog fennel 

Italian ryegrass 
Johnson grass 
Oxalis (red) 
Pineapple weed 
Poison oak 

Ripgut grass 
Russian knapweed 
Sand bur 
Soft chess 
Tansy ragwort 
Wild barleys 
Wild oats 

2,4-D is Dangerous to Crops 

Experiments to date show that of the crop plants, only the cereals and 
other grasses are resistant to 2,4-D. Apparently most other crop plants 
are injured by contact with the chemical, some of them seriously. For 
this reason, the use of 2,4-D has so far been limited to control of weeds 
in grain fields, grass pastures, and lawns, and to ridding soil of weeds 
well in advance of planting crops. 

As experiments with 2,4-D continue, changes are being made in 
recommendations for its use. Consequently, the present recommenda- 
tions must be regarded as tentative. 

General Facts about 2,4-D 

1. 2,4-D acts slowly and two weeks or a month may elapse before the 
treated plants artually die. 

2. Two or more sprayings are usually necessary to eflFect a complete kill of 
many perennial weeds. 

3. Temporary soil sterilization results from use of 2,4-D. How long soil 
remains sterile depends upon amount of chemical used, temperature, 
rainfall or irrigation, soil type, and crop planted. 

4. Sprayers or other equipment used for 2,4-D must be thoroughly cleaned 
before being used for other material. 

5. Spray or dust must never be allowed to reach ornamental or susceptible 
crop plants. Even small amounts of drift are highly injurious. 

6. The acid 2,4-D and its compounds are nonpoisonous to humans and 
animals, noncorrosive, and neither explosive nor inflammable. 



Certain so-called ' 'growth-regulating" compounds have found use as weed 
killers. Chief of these is 2,4-D (or 2,4-dichlorophenoxyacetic acid). Weed 
killers containing this chemical as the effective ingredient are known by many 
trade names. At present over sixty commercial products containing 2,4-D are 
registered with the Bureau of Chemistry, California State Department of 

The earliest work with the material was carried on in the eastern United 
States and in England. Extensive experiments and tests have been conducted 
in California and elsewhere, and much more study is in progress. At present, 
results are far from complete ; and recommendations for use are being con- 
stantly revised as new information becomes available. 

Although 2,4-D shows great promise as an economical herbicide, it has 
sometimes been misused and some injury to crop and ornamental plants has 
resulted. Yet 2,4-D, if used with care and discretion, will undoubtedly find 
a permanent place in weed control. 

Commercial Preparations of 2,4-D 

The most common forms of 2,4-D are the acid (2,4-dichlorophenoxyacetic 
acid) and the salts and esters of this acid. The term 2,4-D applies both to the 
parent acid and its derivatives. 

The Acid. The acid itself is a dry material almost insoluble in water. It is 
usually mixed with a liquid carrier that will keep it in solution when diluted 
with water. Such liquid preparations may differ in the amount of 2,4-D they 
contain, and in the carrier used. 

The Salts. Salts of the parent acid, such as sodium or ammonium salt, are 
dry powders readily dissolved in water, and easily prepared. One dry material 
now on the market contains 60 per cent of the sodium salt of the acid. A wet- 
ting agent is also included in the preparation. This makes it possible to cover 
the plants more evenly with the spray. Roughly, 1% pounds of a 60 per cent 
sodium salt is equal to 1 gallon of liquid preparation having a 9.6 per cent 
acid content. Several of the newer liquid preparations contain alkinolamine 
salts, such as triethanolamine and others, which also wet leaves evenly. 
The Esters. The esters used in the commercial 2,4-D products dissolve more 
slowly in water than do the salts discussed above. They will, however, dissolve 
easily in oil. For this reason, the esters are usually sold in an oil preparation 
which mixes readily with water to form an emulsion. (The esters now available 
in California are methyl, ethyl, and butyl, with isopropyl probably available 
soon.) The content of 2,4-D in these newer preparations is 11 to 36 per cent, 
depending upon the particular product. The number of pounds of 2,4-D per 
gallon in any of the liquid preparations depends upon the strength of the 
2,4-D (the per cent), and the weight per gallon. For this reason, the products 
cannot be compared on the basis of per cent 2,4-D alone. Sometimes the label 

^ Associate in Botany and Associate in the Experiment Station. 
® Professor of Botany and Botanist in the Experiment Station. 


4 California Agricultural Extension Circular 133 

states the actual pounds or ounces of 2,4-D contained. This information helps 
to determine the amount to use, since recommendations are usually made 
in pounds of 2,4-D per acre. 

Other preparations, some of which will undoubtedly contain higher per- 
centages of 2,4-D, are likely to appear on the market. 

So far, tests have shown that the various commercial 2,4-D products are 
about equally effective when compared on the basis of the actual amount of 
acid or acid derivative they contain. Thus in deciding which product to buy, 
prices should be compared on the basis of 2,4-D content. Ease of handling 
and mixing are other factors to be considered — a slightly faster mixing time 
may compensate for a slightly higher price. 

The acid 2,4-D and its compounds are nonpoisonous, noncorrosive, and 
neither explosive nor inflammable. 

How 2,4.D Kills Plants 

This new chemical, which acts differently from other weed killers, was first 
used in very weak concentration to regulate the growth of plants. Later, when 
stronger concentrations were found to kill certain plants, its use as a weed 
killer was developed. 

Plants sprayed with 2,4-D react in various ways. The first noticeable effect 
is in the stems and leaves, which twist and bend, the stems sometimes forming 
loops and coils. In some plants, the stems and leaves dry until the tops are 
completely dead; in others, the stems remain green for several weeks, but 
may swell, develop cracks, and form callus tissue. Sometimes thick pads of 
tissue develop along stems and at the joints. Often, numerous watery, trans- 
lucent buds appear at the crown but do not grow into new shoots. Before 
dropping, leaves of some woody plants change their color to red or yellow, 
as though it were autumn. Several weeks after treatment, seriously affected 
plants may show spongy, enlarged roots. The outer portion of the root may 
slough off and leave wet, stringy cores that will later dry up or rot. 

On most weeds, 2,4-D acts more slowly than other weed killers. It may 
require from four to eight weeks for the weeds to die down completely. The 
effects of spraying show up more rapidly in hot than in cool weather, but 
the end result is the same. 

Plants that form rosettes are especially susceptible in the rosette stage. 
Other plants should be young and growing vigorously, with a well-developed 
leaf surface. Old, mature plants respond slowly or not at all. All plants are 
more easily killed as small seedlings, if application is made at that stage. 

In general, broad-leaved plants are relatively susceptible to 2,4-D, but there 
are exceptions. For example, it is usually easier to effect a permanent kill of 
broad-leaved annuals than of broad-leaved perennials. 

Effects of 2,4-D on Perennial Weeds 

Among perennial weeds, these results have been observed in California : 
Russian knapweed : Variable — per cent of kill ranges from 95 to 10. Should 
be treated in tlie early rosette stage. Difficult to wet, and spray may require 
a wetting agent. 

Klamath weed: Early sprayings not too effective. Better results if plants are 
treated when well developed, but before bloom stage. 

2,4-D AS A Weed Killer 

Fig. 1. — Injury of Tokay grapes from an application of 2,4-D in the vineyard for 
the control of dogbane. Left, longitudinal section through old trunk shows develop- 
ment of callus tissue ; right, new shoot shows distorted leaves. 

Poison oak and wild blackberry: Not seriously affected by a single treatment 
in early spring. Later applications, following complete leafing, look more 
promising. Both plants classified as resistant. 

Dandelion: Highly susceptible. One application usually kills entire infes- 

Cattail, tule, bur-reed, and kelp: Proper applications effective even when 
plants are rooted below the water surface. Cattails become resistant by early 
summer. Three gallons of Diesel oil added to each 100 gallons of spray help 
in penetrating the waxy surface of the plants. Ester preparations are espe- 
cially effective on these species. 

Water hyacinth, yellow water- weed, and hydrocotyle : These floating water 
weeds are easily destroyed by spraying their above-water portions. 
Nutgrass (which is not a true grass, but a sedge) : Somewhat susceptible. 
Treatments must be repeated to destroy underground nuts where food is stored. 

6 California Agricultural Extension Circular 133 

In consulting lists (cover and p. 2), it would be well to bear in mind that 
several factors influence the success of any treatment. Among those to be 
considered are: susceptibility of the weed; stage of growth; amount of leaf 
surface ; ease of wetting ; kind and strength of weed killer used ; and weather 
conditions. With further field experience and additional tests, revision of the 
list may be required. 

Effects of 2,4-D on Grasses 

Since members of the grass family are more resistant to 2,4-D than are 
broad-leaved plants, the chemical is being widely used as a selective spray in 
grain fields. 

Barley and wheat : The usual rate of application has been % to % pound of 
2,4-D per acre, applied in 100 to 200 gallons of water, with a ground rig. 
Although some injury is noted when very young plants are treated, no dam- 
age results when applications are made on grain 4 to 6 inches high. Wild 
radish, mustards, fiddle-neck, and star thistle are readily killed in the young 
stages by applications that ordinarily will not damage grain. 
Corn and milo : One spraying before the crop plants cover the rows will greatly 
reduce the infestation of such perennials as wild morning-glory and kelp for 
the season. One to 1% pounds of 2,4-D per acre is the recommended amount. 
Rice: In rice fields, airplane applications of 2,4-D have been successful in 
control of arrowhead lily, water plantain, burhead, certain sedges, and other 
water weeds. The usual treatment was 15 gallons per acre of a solution con- 
taining 1 to 1% pounds of 2,4-D. (One pound is probably sufficient.) Where 
the water was low when spray was applied, there was some damage, but fields 
sprayed when the water was up and the plants well established showed no 
serious injury. 

Grass pastures, turfs, and grass seed fields : These areas may be treated with 
1% pounds of 2,4-D per acre, in 100 to 200 gallons of water. If the grasses are 
in the seedling stage, however, not more than % pound of the acid should be 
used. Grass seed fields should not be treated when plants are in bloom. In 
mixed pastures, 2,4-D will seriously injure or kill clovers, filaree, and other 
broad-leaved forage plants. 

Lawns : The correct solution is about %o ounce of the parent acid in 5 gal- 
lons of water per 1,000 square feet of lawn (1^/2 pounds in 200 gallons of 
water per acre). Treatment has been very effective in controlling dandelion, 
plantain, chickweed, bur clover, green oxalis, pennywort, heal-all, mouse-ear 
chickweed, and speedwell. Bluegrass and ryegrass are more resistant than 
bent grasses and red top. The spray is not effective against Bermuda grass, 
crab grass, and red oxalis — ^weeds common in bluegrass lawns. 

Use of 2,4-D in Orchards 

In orchards, 2,4-D has been used to control wild morning-glory and other 
perennial weeds, except grasses. Thus far, no serious injury has been reported. 
Drift has resulted in a slight amount of curling and discoloration of young 
growth, but there was no stunting of tree growth or reduction in yield and 
quality of fruit. If 2,4-D is used in orchards, however, care should be taken to 
reduce drift to a minimum, and to avoid any undue accumulation of the 
chemical in soil areas resulting from spilling or careless application. 

2,4-D AS A Weed Killer 

Effect of 2,4-D on Soil 

In some instances, the use of 2,4-D has resulted in sterilization of the soil. 
Some crops, such as broccoli, cabbage, sugar beets, tomatoes, beans, and root 
crops have been damaged when grown in fields where 2,4-D had been used (see 
fig. 2). However, in a number of cases, extra large amounts of the chemical 
had been applied, and most of the fields 
had remained dry from the time of ap- 
plication until just before the crop was 

Tests have shown that 2,4-D leaches 
out of warm, moist soils in thirty to 
sixty days, but it may remain in cool, 
dry soils for six months or longer. Tests 
also indicate that more 2,4-D is re- 
tained by heavy than by light soils. 
Flood irrigation after an application 
of 2,4-D will help remove it, especially 
in summer when the soil is warm. In 
many areas, soil is safe for spring or 
summer planting after the winter 
rains. If areas are treated during the 
late spring or summer, however, it is 
not wise to plant susceptible crops un- 
til several months have passed. 

The Use of 2,4-D Dust 

Tests to date indicate that while 2,4-D 
applied as a dust is effective in killing 
weeds, slightly more 2,4-D per acre 
may be required than when applied as 
a spray. Until some satisfactory method 
is developed for control of drift toward 
susceptible crop plants, it will be dan- 
gerous to use dust near any crops ex- 
cept grain. 

Tests in which dry 2,4-D was applied to the soil at the time of seeding grains, 
to destroy young germinating weed seedlings, have been disappointing. Grain 
was injured much worse than by spray treatments. Success of dry applications 
to the soil depends upon the amount of rainfall and, under California condi- 
tions, cannot yet be recommended. 

The Best Time to Spray 

All available information indicates that weeds should be sprayed while still 
young and growing vigorously. If plants are sprayed when they are old or 
near maturity, the chemical will have a slow, uneven reaction. Although most 
weeds should not be sprayed when temperature is low, certain early spring 
weeds are making vigorous growth at this time, and applications of 2,4-D give 
satisfactory results. 

Fig. 2. — Eoot systems of onion bulbs: 
The upper root system was grown in a solu- 
tion containing 1 part per million 2,4-D 
(.0001 per cent) ; the lower, in 5 parts per 
million 2,4-D (.0005 per cent). 

8 California Agricultural Extension Circular 133 

Cost of Treatment 

These weed-killing substances are usually applied as a spray. If an airplane 
is used, only about 15 gallons per acre are required, as compared with 100 or 
more gallons per acre when a ground rig is employed. The amount of 2,4-D 
used depends upon the thickness of the weed growth, but ranges from % to 
3 pounds per acre. The following figures are average costs per acre for some 
of the commercial products: for selective weed control in grain, % pound 
per acre, $2 to $3 ; for morning-glory, 1% pounds per acre, $4 to $6 ; and for 
some of the more resistant weeds, $8 to $12. Prices may vary in different 
localities, depending upon amounts available and quantity purchased. 

In addition to the cost of the chemical there is the cost of application. This 
is about $2 per acre, but varies with type of equipment and amount of acreage 
sprayed. If a satisfactory method of applying dust is developed, the over-all 
cost of treatment may be reduced. 

Use 2,4-D with Caution 

1. Do not expect miracles. 2,4-D is a new material, not thoroughly tested. 
Its action is slow, sometimes requiring a month or longer to kill the tops and 
roots of the weeds, especially perennials. Two sprayings may be necessary 
because some plants are missed during the first spraying, and some new plants 
may come up from lateral roots which did not die. Watch the sprayed area 
closely and spray new growth or regrowth as soon as it is large enough. 

2. Soil sterilization results from use of 2,4-D. How long the effects will re- 
main depends upon amount of chemical used, temperature, rainfall or irriga- 
tion, soil type, and crop planted. While grains and grasses apparently suffer 
no damage if planted within a few weeks after spraying, beans, peas, lettuce, 
tomatoes, cabbage, broccoli, sugar beets, alfalfa, and many other crops are 
extremely sensitive to small quantities of the chemical. 

3. A sprayer or any other equipment which has contained 2,4-D must be 
thoroughly washed before it is used for other material. Otherwise, field, 
orchard, and ornamental plants may be damaged if even a small amount 
remains in the sprayer. One cold-water rinse is not sufficient. Use several 
changes of water (preferably warm) to which a little baking soda or washing 
soda has been added. 

4. When spraying a lawn or other area, never allow the spray to reach 
near-by ornamental or crop plants. Even small amounts of drift will injure 
these plants, some of which are highly sensitive. 

Recently, grapevines from the San Joaquin Valley have shown what ap- 
peared to be damage from 2,4-D after treatment of morning-glory in the 
vineyard. The effects on the vines may have been due to drifting of spray, to 
action through soil, or both. Experience with it in vineyards is not extensive 
enough, however, either to encourage or discourage its use for weed control 
in such plantings. Other similar cases of injury to near-by crop plants by 
spray have been reported. This past season, beans next to a rice field which 
was sprayed from the air were injured by drift.